Superior binders for photoconductive layers containing zinc oxide



United States Patent ()fitice 3,132,941 Patented May 12, 1964 3,132,941 SUPERISR BENDERS FGR PHUTOCONDUCTWE LAYERS CONTENG ZlNC OXIDE Frederick A. Stahly, Rochester, N.Y., and Kenneth Qarl Herr, Berkeley, Caiii, assignors to Eastman Kodak Company, Rochester, N.Y., a corporation of New Jersey No Drawing. Filed Dec. '2, 1960, Ser. No. 73,134 2 Claims. (til. 961) This invention concerns coating for use in xerographic and conductographic electrophotographic processes, more particularly, xerographic coatings of improved flexibility, electrical properties, color stability and photoconductographic coatings with improved moisture resistance.

In one embodiment of the xerographic process, a coating containing zinc oxide or a similar material in a suitable resinous binder is provided with an electrostatic charge and then exposed to a light image in a manner similar to that employed with ordinary silver halide photographic material. The charge is dissipated in the areas where the light strikes the zinc oxide-resin coating. Subsequently, a colored toner powder is caused to adhere to the surface by electrostatic attraction in those areas which have not been exposed to the light and in which residual charge remains.

To satisfy the electrical requirements of the xerographic process the coating of zinc oxide and resin must show good resistivity in the dark and adequate conductivity when exposed, and the resistivity of the unexposed region must not be seriously reduced at high relative humidities. At the extreme end of the relative humidity range, i.e., above about 90%, some zinc oxide-resin xerographic layers have low charge storage capacity and rapid dark decay rates.

Since the xerographic prints are intended for use in a wide variety of ways, it is important that the combination of support, e.g. paper, and the coating have good flexibility, satisfactory color, and good stability. Coatings of zinc oxide in a styrene-butadiene copolymer binder have, for example, become yellow and brittle on keeping even in the absence of strong light. Some other binders which have been used in the past tend to be brittle and will crack under extreme flexing.

Photoconductography is drescribed in detail in British Patent 188,030, Von Bronk, and British Patent 464,112, Goldman. British Patent 789,309, issued to Berchtold, describes an improvement on the process using a protective layer between photoconductor and recording layer and Belgian Patent 561,403 issued to Johnson et a1. describes, in considerable detail, systems using zinc oxide asthe photoconductor.

A satisfactory binder has also been needed for use in photoconductographic coatings. The light-sensitive material can comprise a conducting support such as a metal foil laminated to an underlying paper base, coated wtih a zinc oxide-resin photoconductive layer. Since processing is carried out with aqueous solutions of electrolytes, it is important that the combination of support and the coating be reasonably moisture resistant.

We have found that the combination of certain resinous materials as a binder for the zinc oxide results in a coating for paper and other supports to be used in the xerographic and conductographic electrophotographic processes which has superior properties both for the photographic purposes and for physical handling.

One object of this invention is to provide a photographic coating for use in an electrographic process having improved electrical properties. An additional object is to provide a process for manufacturing an electrographic photographic paper which has improved physical properties such as good flexibility, satisfactory color and good stability.

Another object is to provide a binder for use with zinc oxide, cadmium sulfide or the like for coating on a conducting substrate such as paper, electrical-conducting glass, aluminum foil laminate, and the like. A further object is to provide a process for making a photoconductive element having improved electrical properties and improved moisture resistance.

Our invention comprises a mixture of a silicone resin and a styrene-butadiene resin as a carrier for zinc oxide. The proportions of butadiene-styrene resin to silicone resin may range from 95:5 to 50:50 in order to achieve the improved results, the preferred range being from 95 :5 to :20.

The butadiene-styrene resins are well known in the art; see Zwicker, Ind. Eng. Chem, 44, 774786, especially pages 778-779, and Bovey et al., Emulsion Polymerization, lnterscience Publishers, Inc, N.Y., 1955, pp. 406-7. The butadiene-styrene resins useful in our invention are those in which butadiene and styrene are polymerized in a ratio of butadiene to styrene of 60:40 to 20:80, although the preferred ratio is from 45 :55 to 30:70.

The silicone is an organo polysiloxane resin having the following the general formula:

where x is an integer from 6 to 40, and R is a methyl or phenyl radical, so chosen that both radicals on any given silicon atom are identical and the molar ratio of methyl to phenyi radicals varies from 4:1 to 1:4. Higher molecular weight polysiloxanes are operable for use in this invention as long as they are sufficiently compatible with the styrene-butadiene resin to permit coating.

The silicone resins may be obtained by the catalytic equilibration procedure disclosed in chapter 6 of E. G. Rochows Introduction to the Chemistry of the Silicones, John Wiley and Sons, Inc, New York, Second Edition, 1951. Mixtures of dichlorodiphenylsilane and dichlorodimethylsilane are treated with the appropriate quantity of Water to hydrolyze the above chlorosilanes with concurrent polymerization. For our purpose, the mixture composition may vary from a ratio of 20 molar percent of dichlorodiphenylsilane and 80 molar percent of dimethyldichlorosilane to a ratio of 80 molar percent dichlorodiphenylsilane and 20 molar percent dichlorodimethylsilane although the preferred mixture is 1:1 on a molar basis.

The zinc oxide is used in a ratio of from less than 0.5 :1 up to about 10:1 zinc oxide to binder.

The coating composition is normally prepared by dispersing the zinc oxide in a solvent followed by the addition of solvent solutions of the resinous materials.

In the practice of this invention, as it pertains to xerography any charge-storing zinc oxide can be used in preparing the photoconductive layer to which the invention pertains. As is known in the art, there are essentially two types of zinc oxide, the charge-storing type and the noncharge-storing type. It is the charge-storing variety of zinc oxide Which can be advantageously employed in the present invention. 'Among the charge-storing varieties of zinc oxide there are (a) Type F, or, as it is sometimes referred to, French process or indirect-process zinc oxide; (b) pink zinc oxide prepared as described by S. M. Thomson in US. Patents 2,727,807 and 2,727,808, issued December 2-0, 1955; and (c) wet process zinc oxide described in page 771 of RCA Review 20: No. 4, December 1959.

Its identifying characteristic is the ability, when in a suitable vehicle or binder, to store electric charge. It is normally referred to as charge-storing zinc oxide. This is a simplified term since it is not the Zinc oxide crystals or particles by themselves which have this characteristic but rather the Zinc oxide particles in a binder. As far as the present invention is concerned it does not matter how the charge-storing zinc oxide is made.

In the practice of this invention as'it pertains to photoconductographic processes, any zinc oxide having sulficiently high photoconductivity with concomitantly suiiiciently low dark current may be used. French process zinc oxides generally meet the requirements. A particularly useful one is XX-78 zinc oxide sold by the New Jersey Zinc Co.

The following examples are intended to illustrate our invention but are not intended to limit it in any way.

Example 1 A dispersion was prepared by mixing in an electrically rotating blender for ten minutes the following:

' Grams Butadiene-styrene copolymer (45:55) 14 Zinc oxide 56 Xylene 100 Methanol 2 The dispersion was coated to attain a dry thickness of about 0.001 inch onto a clean 0.10 thick aluminum film. The dry coating was exposed to the optical image formed by projecting a line negative, and developed immediately after exposure with a sponge electrode connected to the positive terminal of an 80 volt source of DC. potential. The aluminum foil was connected to the negative terminal. The sponge electrode was wetted with an electrolytic developing solution consisting of nickel chloride and sodium thiosulfate in water.

The resulting print had acceptable flexibility and excellent abrasion resistance. There were numerous dark spots developed up in the unexposed background areas as a result of electrical breakdown, or shorting. The material was assigned an arbitrary relative photographic speed value of 100. On keeping, the background yellowed badly and the coating became less flexible, particularly in samples exposed to strong sunlight and/or elevated temperatures.

Example 2 The example was prepared and tested like Example 1 except that a polysiloxane silicone resin (a mixture of polysiloxanes H[OSi(CH OSi(C l-l ]OH where n is 3-20) was used in equal weight quantity to replace the styrenebutadiene copolymer resin.

The resulting coating was soft, and its abrasion resistance was poor. Relative speed of the print was less than 1. However, the electrical characteristics were better in that no shorting occurred as evidenced by a total absence of black spots in the unexposed background. On keeping, the background showed no tendency to discolor.

Example 3 Quantities of the dispersions made according to Examples l and 2 were mixed together to give three compositions, A, B and C, with the following ratios of styrenebutadiene copolymer to silicone resin by weight A.1t01 B. 4to1 C.14to1 These composition swere coated, dried and tested like Example 1.

The excellent abrasion resistance of Example 1 was retained and the flexibility was improved in all three coatings of Example 3 and the speed increased from Examples 3A to 3C, approaching the very high speed of Example 1. Even coating A, which represented 50 percent silicone in the polymer mixture showed no detectable-deterioration in abrasion resistance and flexibility. At the lowest concentration of silicone, around 6 percent (coating C), the improvement in electrical properties and the protection from background yellowing were maintained.

A dispersion was prepared by mixing in an electrically rotating blender for ten minutes the following:

Grams Butadiene-styrene copolymer (45 :55) 14 Zinc oxide 56 Xylene 100 The dispersion is coated on paper to obtain a dry thickness of about 0.001". Upon charging, this layer reaches a saturation potential of 400 volts. The charged member is exposed to a line or halftone image (either negative or positive) and developed according to the procedure described in RCA Review 15: pages 469484, December 1954.

The resulting print had acceptable flexibility and excellent abrasion resistance. At high relative humidities (about percent RH) the layer had low charge storage capacity and rapid dark decay rate yielding a print of poor contrast and very low density. On keeping, the backgrounds yellowed badly, and the coating became less flexible particularly in samples exposed to strong sunlight and/ or elevated temperatures.

Example 5 This example was prepared and tested like Example 4 except that the polysiloxane silicone resin of Example 2 was used in equal weight quantity to replace the styrenebutadiene copolymer resin of Example 4.

The resulting coating was soft and its abrasion resistance was poor. However, the electrical characteristics were better in that at higher humidities the layer held its charge. On keeping, the background showed little or no tendency to discolor.

Example 6 Quantities of the dispersions made according to Examples 4 and 5 were mixed together to give three compositions, A, B and C, with the following ratios of styrenebutadiene copolymer to silicone resin; by weight:

A. 1 to 1 B. 4 to 1 C. 14 to 1 These compositions were coated, dried and tested like Example 4.

The excellent abrasion resistance of Example 4 was retained and the flexibility was improved in all three coatings of Example'6. Even coating A, which represented 50 percent silicone in the polymer mixture showed little or no detectable deterioration in these properties' At the lowest concentration of silicone, around 6 percent (coating C) the improvement in electrical properties at high humidity and the protection from background yel lowing were maintained.

Optionally the zinc oxide may be sensitized by the addition of a sensitizing dye to the mix. Typical dyes are disclosed in the RCA Review, 15: pp. 469-484, December 1954, British patent application 811,165 and pending U.S. applications 630,461, 630,462, 630,463, 645,415 and 24,122.

Acetone, Solvesso 100, 50:50 xylene-hexane, methyl ethyl ketone, dioxane, methyl isobutyl ketone, xylene, toluene, benzene and similar solvents may be used in Examples 3 and 6 provided they are mutual solvents for both the silicone and the butadiene-styrene copolymer and do not disturb the electrical properties; these solvents are intended when volatile organic solvents are referred to herein. It is further understood that non-solvents may be added to the solvent mixture within the scope of our invention in order to improve coating properties, viscosity and the like.

Other conducting salts may also be used as well as zinc oxide. Such salts include cadmium sulfide, selenium 53 salts, and the like which are known in the arts as photoconductors.

The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention as described hereinabove and as defined in the appended claims.

The ratios referred to herein are by weight unless otherwise designated.

We claim:

1. An electrophotographic layer comprising a support having coating thereon a composition consisting of 64.5 to 4.5% by weight of a copolymer composed of from 20-60% by weight butadiene and 8040% by Weight styrene, 0.5 to 33% by Weight of a silicone resin having the formula:

R H/-O S i OH wherein x is an integer from 6 to 40 and R is selected from the group consisting of methyl and phenyl radicals, so chosen that both radicals on any given silicone atom are identical and the molar ratio of methyl to phenyl radicals is 4:1 to 1:4, and 33 to 91% of charge-storing zinc oxide.

2. An electrophotographic layer comprising a paper support having coated thereon a composition essentially consisting of 64.5 to 4.5 by weight of a copolymer composed of butadiene and styrene having a ratio of 45 to 30 parts by weight butadiene to 55. to 70 parts by weight styrene, 8.5 to 33% by weight of a silicone resin having the formula H[()Si(CI-I OSi(C H OH where n is 3 to 20, and 33 to 91% charge-storing zinc oxide.

References Cited in the file of this patent UNITED STATES PATENTS 2,875,054 Griggs et a1. Feb. 24, 1959 2,937,944 Van Dorn et al. May 24, 1960 2,990,279 Crumley June 27, 1961 FOREIGN PATENTS 565,863 Canada Nov. 11, 1958 

1. AN ELECTROPHOTGRAPHIC LAYER COMPRISING A SUPPORT HAVING COATING THEREON A COMPOSITION CONSISTING OF 64.5 TO 4.5% BY WEIGHT OF A COPOLYMER COMPOSED OF FROM 20-60% BY WEIGHT BUTADIENCE AND 80-40% BY WEIGHT STYRENE, 0.5 TO 33% BY WEIGHT OF A SILICONE RESIN HAVING THE FORMULA: 